High-temperature ovens, called reactors, are used to process semiconductor wafers from which integrated circuits are made for the electronics industry. A circular wafer or substrate, typically made of silicon, is placed on a wafer support called a susceptor. Both the wafer and susceptor are enclosed in a quartz chamber and heated, typically by a plurality of radiant lamps placed around the quartz chamber. A reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes in other equipment, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits.
If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure.
Various CVD process parameters must be carefully controlled to ensure the high quality of the resulting semiconductor. One such critical parameter is the temperature of the wafer during the processing. The deposition gas reacts at particular temperatures and deposits on the wafer. If the temperature varies greatly across the surface of the wafer, uneven deposition of the reactant gas occurs.
In certain batch processors (i.e., CVD reactors which process more than one wafer at a time) wafers are placed on a relatively large-mass susceptor made of graphite or other heat-absorbing material to help the temperature of the wafers remain uniform. In this context, a "large-mass" susceptor is one which has a large thermal mass relative to the wafer. The thermal mass of a solid, or its lumped thermal capacitance, is given by the equation: EQU C.sub.T =.rho.Vc
where:
.rho.=the density of the solid, PA1 V=the volume of the solid, and PA1 c=the specific heat (heat capacity) of the solid. PA1 C.sub.T,eff =effective thermal mass of the susceptor [J/.degree. K.] PA1 D=diameter of the wafer [m], and PA1 x=multiplication factor [J/.degree. K./m.sub.2 ], PA1 t=thickness of the susceptor [m], and PA1 c=specific heat (heat capacity) of the solid [J/.degree. K.].
Thus, the thermal mass is directly related to its mass, which is equal to the density times volume and to its specific heat.
One example of a large-mass susceptor is shown in U.S. Pat. No. 4,496,609 issued to McNeilly, which discloses a CVD process wherein the wafers are placed directly on a relatively large-mass slab-like susceptor and maintained in intimate contact to permit a transfer of heat therebetween. The graphite susceptor supposedly acts as a heat "flywheel" which transfers heat to the wafer to maintain its temperature uniform. The goal is to reduce transient temperature variations around the wafer that would occur without the "flywheel" effect of the susceptor.
Although large-mass susceptors theoretically increase temperature uniformity across the wafers during steady state, the large thermal mass of the susceptor makes it slower to respond to temperature transients than the wafer, resulting in instances where the temperatures of the two elements are different. This is highly undesirable because during temperature transients, the wafer temperature does not correspond to the temperature of the susceptor, and the process becomes difficult to control. Temperature nonuniformities will easily occur across the wafer surface during these transients.
In recent years, single-wafer processing of larger diameter wafers has grown for a variety of reasons including its greater precision as opposed to processing batches of wafers at the same time. Typical wafers are made of silicon with one common size having a diameter of 200 mm and a thickness of 0.725 mm. Assuming a density of silicon of 2330 Kg/m.sup.3, and a specific heat of 913 J/kg/.degree. K. at 800.degree. K., the thermal mass of such a wafer used in single-wafer processing is approximately 48 J/.degree. K. Recently, larger silicon wafers having a diameter of 300 mm and a thickness of 0.775 mm have been proposed, as they even more efficiently exploit the benefits of larger single-wafer processing. The thermal mass of 300 mm wafers is approximately 117 J/.degree. K. Additionally, even larger wafers are contemplated for the future.
Although single-wafer processing by itself provides advantages over batch processing, control of the process parameters remains critical and is perhaps more so because of the increased cost of the larger wafers. One example of a single-wafer processor is shown in U.S. Pat. No. 4,821,674, which utilizes a circular rotatable susceptor having a diameter slightly larger than the wafer. This susceptor is preferably made of graphite and has a much lower thermal mass than the aforementioned slab-type batch processing susceptor. One of the advantages of a reduced thermal mass is a reduced cycle time and increased throughput. Nevertheless, the thermal mass of a production version of the susceptor described in U.S. Pat. No. 4,821,674 is much larger than the thermal mass of the wafer, which may lead to temperature differences between the wafer and susceptor from thermal transients during the process. In these systems, moreover, the temperature at points around the susceptor and wafer should be monitored, which requires complex thermocouple or pyrometer apparatusses.
The heat transfer between the susceptor and wafer depends on the geometrical projection of the surface area of one body on the other. Thus, the area directly underneath the wafer is best used when calculating and comparing susceptor thermal masses. The thermal mass of the region of the susceptor directly underneath the wafer will be termed the "effective" thermal mass. The peripheral area outside this region affects heat transfer between the susceptor and wafer to a lesser extent, although large edge losses can create unwanted temperature differentials in the susceptor. The effective thermal mass for a standard disk-shaped susceptor of varying sizes can be expressed as a constant x times the square of the diameter of the wafer being supported. In other words, from the above equation for thermal mass the following ratio can be derived: EQU C.sub.T,eff =xD.sup.2
where:
where: ##EQU1## and where: .rho.=density of the solid [kg/m.sub.3 ],
One graphite susceptor utilized in the single-wafer processing system described in U.S. Pat. No. 4,821,674, for example, has a diameter of 220 mm, a thickness of 6.4 mm, a density of 2250 kg/m.sup.3, and a mass of 0.57 kg. With a heat capacity of 1650 J/kg/.degree. K. at 800.degree. K., the graphite susceptor has an effective thermal mass (i.e., directly under a 200 mm wafer) of approximately 746 J/.degree. K. at 800.degree. K., which is more than fifteen times the thermal mass of the 200 mm wafer. For these susceptors, and using SI units, the multiplication factor x equals about 18,661 J/.degree. K./m.sub.2. Thus, for 300 mm wafers, the effective thermal mass of the graphite susceptor is, EQU C.sub.T,eff =18,661(0.3).sup.2 =1,680 J/.degree. K.
which is more than fourteen times the thermal mass of the 300 mm wafer. (Of course, these numbers will be modified with the use of English units and constants.) These large differences in the thermal masses of the susceptor and wafer mean the susceptor lags behind the wafer significantly during fast heat-up and cool-down cycles.
FIG. 1 illustrates schematically a sequence of heat-up and cool-down of a prior art single-wafer processor circular susceptor and wafer thereon. As illustrated, the wafer has a steeper temperature climb during the heat-up stage so that it reaches and surpasses a steady state temperature well before the susceptor. There may be some overshoot of temperature, which is exaggerated in the drawing, due to a delay in response of the wafer and susceptor to the varying intensity of the radiant heat lamps. Ultimately, the two elements attain a steady state temperature until a cool-down period, whereupon the wafer cools down much faster than the susceptor. Not only do temperature differences create a risk of temperature non-uniformity on the wafer, the process throughput is limited by the time it takes for the susceptor to heat up and cool down. High throughput remains a prime concern in single-wafer semiconductor processing.
The edges of the susceptor will be colder than the center when the susceptor is uniformly irradiated from the top and/or bottom. Edge losses are significant in many thick susceptors due to their large surface area on the edges. This situation is schematically indicated in FIG. 2 which shows relatively large heat losses at the edge of a thick susceptor. The edge surface area of the graphite susceptor used in conventional single-wafer processing is 4.5.times.10.sup.-3 m.sup.2, which is approximately 5% of the total surface area. The temperature nonuniformity of the susceptor from large edge losses may result in a temperature nonuniformity of the wafer, as seen in the graph of FIG. 3, which affects the quality of the resulting semiconductor. Various solutions have been offered to accommodate for the reduced temperature at the edges of the susceptors, including placing structures around the susceptors and modifying the intensity of heat radiated to different areas of the susceptor. All of these are complicated and increase the cost of the resulting processing apparatus.
Consequently, there is a need for an improved susceptor to increase throughput of semiconductor processing devices while ensuring temperature uniformity across the wafer surface.